The automatic air brake systems of railway cars output a brake command at the car control valve for applying the car brakes in response to a decrease of brake pipe pressure from the brake valve of the head car or locomotive. A variable load valve device by which the brake pressure is adjusted to reflect the car load condition may be included in the automatic brake system. Such a variable load valve used in an automatic brake system is, for example, described in Japanese Patent Publication 43-17529 (Tokkosho). This variable load valve 1 is illustrated in FIG. 6, in which 2 is an air passage from the air spring, and 3 is the weighing mechanism, which converts a change of the air spring pressure into a change in the angle of the adjusting rod 4. The rotation of the adjusting rod 4 moves a roller 8 in the right and left directions in the figure via a lever 6 and a linking piece 7. Also in the figure, 9 is the adjusting unit for the brake cylinder pressure; 10 is the air passage connected to a compressed air source; 11 is an air passage having a command air pressure; 12 is an air passage connected to the brake cylinder; 13 is a backflow check valve; 14 and 15 are diaphragm pistons; and 16 is a balancing lever. Variable load valve 1 outputs the brake cylinder pressure through air passage 12 in accordance with the load controlled air spring pressure effective in passage 2. This is done in response to the command air pressure received in air passage 11. That is to say that the balancing ratio between the command air pressure exerted on diaphragm piston 14 acting on one side of lever 6 and the brake cylinder pressure exerted on diaphragm piston 15 acting on the other side of lever 6 changes because the weighing mechanism 3 moves the roller 8 in response to the air spring pressure. Other features include the roller 8 moving toward the left direction in the figure when cars are empty, and toward the right direction when they are loaded. For example, when the cars are loaded, roller 8 is moved to the right, the command pressure acting on diaphragm piston 14 balances with the brake cylinder pressure acting on diaphragm piston 15. Therefore, a higher air pressure than that in an empty car condition acts on the diaphragm 15 and the back-flow check valve 13 opens to supply air from passage 10 to brake cylinder passage 12 by way of diaphragm piston 14.
Another example is seen in the Japanese Patent 62-201557 (Tokugansho) which is different from the automatic brake system described above, in that the arrangement changes the characteristic of air pressure supplied to the brake cylinder in response to changes in the air spring pressure. In this connection, the system has combined features of being able to change the effective area of either the balancing piston (which receives the pressure from the brake cylinder and the intermediate body with air supply hole, valve seat, and air supply valve) or the control piston (which receives the pressure from the air spring).
When a command air pressure is provided for the change from an empty car condition to a loaded car condition, a large amount of pressurized air is supplied to passage 12 of the brake cylinder side via the backflow check valve 13 to increase the brake cylinder pressure. Because of this, the brake force will be obtained corresponding to the increase in the car weight. The amount of lift of the back-flow check valve, at that time, becomes relatively small in response to the movement of the command piston 14, due to the rightward movement of roller 8 changing the effective ratio of balance lever 16 when the weighing mechanism 3 detects a heavy load condition. This requires a relatively longer time to increase the brake cylinder pressure than during light load conditions and, therefore, has the problem of being inappropriate for operating brakes. In addition, another problem arises in that the component parts wear very quickly and tend to have a short life because the structure using the balancing lever 16 supports the control diaphragm piston 14 and the balance diaphragm piston 15, therefore, producing a tremendous force upon the point where the command force is applied, the point where the balance force is used, and at the fulcrum point.